This invention relates to the processing of waste material, and more particularly to improvements in processes for drying or dewatering sludges and a vehicle which may be used in practicing such processes.
Disposition of waste product sludges is a very large and growing problem. For example, the quantity of processed sewage sludge which results from biological treatment of human and industrial waste is presently estimated to be about five million dry tons per year in the United States, and it is estimated that there will be perhaps two or three times this amount when treatment plants presently under construction are in full operation. Added to this quantity are additional large quantities of the following types of sludges: tailings from mineral ore beneficiation; sludges from scrubbing of flue gases; dredgings from navigation channels; and sludges from a variety of industrial processes.
Most processed sewage sludges contain valuable nutrients and will, if applied as a fertilizer to soil, improve soil tilth and water-holding capacity and increase crop yields. However, processed sewage sludge is typically associated with large quantities of inherently attached water, rendering it practically unmanageable for use as a fertilizer. Hence most domestic sludge presently is disposed of by dumping it in rivers, lakes and oceans, or by burying it, or its ashes following incineration, in the earth.
Processes have been developed for separating out the water associated with processed sludge to thereby reduce the volume of the processed sludge and convert it into a salable fertilizer, e.g., heat drying, air drying on solid ground or on sand drying beds, centrifuges, filter presses, vacuum filters, and free drying in cold climates. These sludge drying or dewatering processes have been used with only varying degrees of success and at considerable expense.
Sludge lagoons also have been economically employed to accomplish a gradual thickening of sludge stored therein through the excess of evaporation over precipitation. Drying or dewatering of sludge in sludge lagoons has not been entirely successful, however, in that the evaporation rapidly dries out the surface layer of the sludge, forming a crust which protects the remainder of the sludge from further evaporation except at a much reduced rate.
Vehicles have been developed for moving across the surface of a sludge lagoon, to break up the crust formed on the surface of the sludge, and to agitate the sludge, thereby enhancing evaporation. The operation of these vehicles on sludge lagoons has not been entirely efficacious, however. Sludge typically is of a high viscosity. Prior art vehicle designs have encountered problems because of contact between the sludge and nonrotating parts of the vehicles, resulting in exertion by the sludge upon the vehicle of large shearing forces, hindering movement of the vehicles. Alternately, prior art vehicle designs have resulted in collection of sludge thrown by the rotating parts upon the vehicles, thereby adding to the weight of the vehicles and further hindering their movement. Finally, eventually the thickness of the crust formed is such that either such vehicles cannot be used, i.e., they are unsuitable for operation upon a "solid" surface, or the effective depth of agitation of such vehicles will not penetrate beyond the thickness of the crust. Further processing of the sludge thence requires harvesting of the crust from the surface of the lagoon, with different machinery, which is either impossible or impractical, and in any event, adds to the cost of the overall operation.
Accordingly, it is an object of the present invention to provide an improved process for dewatering and thickening of processed sludge.
More specifically, it is an object of this invention to provide a process for dewatering processed sludge in a sludge lagoon whereby moisture may be continually removed from the sludge in the lagoon by continually exposing wet sludge within the lagoon to the natural evaporative capacity of the atmosphere.
It is another object of this invention to provide a process whereby dry sludge which forms in a layer upon the surface of a lagoon may be continually removed so that it does not become impenetrable and so that wet sludge therebeneath is easily exposed to continue the drying process.
It is another object of this invention to provide a process for dewatering processed sludge in a sludge lagoon whereby the deleterious effect of precipitation on the drying of the sludge is substantially reduced.
Another object of this invention is to provide an improved vehicle for use in dewatering processed sludge.
Further, it is an object of this invention to provide a vehicle which can be operated on material of varying moisture content.
It is a further object of this invention to provide an improved vehicle for economical agitation of sludge in sludge lagoons and for harvesting of dried sludge, without requiring separate equipment for the different operations.
Moreover, it is an object of this invention to provide a vehicle for use in dewatering processed sludge which is adapted for movement on a sludge lagoon containing sludge of any moisture content and which may be employed to systematically shift and harvest controllable amounts of dry sludge on the surface of the lagoon and to agitate the remaining sludge.
It is a more specific object to provide a vehicle which is adapted for movement on a sludge lagoon containing sludge of any moisture content and which may be employed to agitate, shift and harvest sludge in the lagoon without the deleterious problems of prior art vehicles.
Other objects, advantages and capabilities of the present invention will become apparent as the description thereof proceeds.
A process is provided for accelerated drying or dewatering of sludge, along with a vehicle which may be used in practicing such process. Sludge of a relatively high moisture content is introduced into a sludge lagoon. A crust of relatively dry sludge is rapidly formed on the surface of the sludge by the natural evaporative capacity of the atmosphere. The sludge is agitated to break up and depress the crust of relatively dry sludge and to bring wet sludge from beneath the crust to the surface and thereby to expose the wet sludge to the natural evaporative capacity of the atmosphere, to thereby develop additional thickness of relatively dry sludge. The relatively dry sludge can also be systematically shifted laterally in such a manner as to tilt the surface of the lagoon and facilitate drainage of precipitation which collects on the surface of the sludge. A controlled amount of the relatively dry surface sludge is harvested by removal from the lagoon, for use as fertilizer or other beneficial use, while continuing the agitation of the remaining material to continue the development of a substantial thickness of relatively dry sludge.
A vehicle for carrying out the agitation and harvesting is supported in sludge of varying moisture contents by buoyant drive wheels which assure a depth of penetration of the surface such that only its wheels are in contact with the sludge. The wheels of the vehicle are both floats and traction units and are designed to agitate the sludge as the vehicle passes over it. Equipment is mounted on the front end of the vehicle for skimming off a controllable depth of undisturbed relatively dry surface sludge ahead of the agitating wheels and conveying this drier material to a point whence it is either discharged to one side or the other of the vehicle, transported to the side of the lagoon, or loaded onto the vehicle.
For a more complete understanding of this invention, reference should now be had to the embodiments illustrated in the accompanying drawings and described below by way of examples of the invention.